This invention relates to radio frequency circuits and more particularly to radio frequency calibration circuits.
As is known in the art, there exists a trend toward operation of radar systems at higher frequency bands particularly the millimeter wave frequency band. For example a typical monopulse radar system includes an antenna, coupled to a monopulse arithmetic circuit (hereinafter monopulse circuit). The antenna feeds radio frequency (RF) signals to input ports of the monopulse circuit. The monopulse circuit provides at its output ports so-called monopulse output signals. The monopulse output signals typically include a sum signal, an elevation difference signal and an azimuth difference signal. The monopulse circuit provides the monopulse output signals having a particular amplitude and phase characteristic. Moreover, the amplitude and phase characteristics of each of the monopulse output signals have a particular amplitude and phase relationship with respect to each other. The monopulse circuit feeds such monopulse output signals to an RF receiver.
The RF receiver typically includes a signal path generally referred to as a receiver channel for each of the monopulse signals provided by the monopulse circuit.
Each receiver channel receives a corresponding monopulse output signal having an RF frequency, and downconverts said monopulse signal to an intermediate frequency (IF). The receiver channels typically include RF circuit components such as limiters, filters, amplifiers, mixers and the like which operate over a desired frequency band. Such circuit components are identically disposed to provide the receiver channels as like receiver channels. One problem with such RF receivers however is that due to variations in electrical characteristics of the circuit components, each of said like receiver channels may have different insertion loss and insertion phase characteristics.
Such insertion loss and insertion phase imbalances between the receiver channels is a source of error in a radar system since the RF receiver feeds the IF monopulse signals to a digital signal processor which provides direction finding information to the radar system by comparing the amplitude and phase of the sum signal to the amplitudes and phases of the elevation difference and azimuth difference signals. Thus to obtain accurate direction finding information the RF receiver should not change the relative amplitude and phase characteristics of the monopulse output signals fed thereto.
One solution to this problem in a conventional monopulse radar system operating in the microwave frequency range is to provide the RF receiver channels with RF circuit components having matched electrical characteristics. That is, bandpass filters, limiters, amplifiers, mixers and the like are provided as matched sets for each of the receiver channels. With matching of components, each receiver channel should provide substantially equal insertion loss and insertion phase characteristics to signals fed thereto.
There are several problems with this approach. For example, in those RF receivers having a plurality of like receiver channels it is relatively expensive to purchase RF circuit components having matched insertion loss and insertion phase characteristics. Moreover, impedance mismatches between components will also provide amplitude and phase variations in the receiver channels. Thus such matched sets of circuit components should be specified having very low reflection characteristics (i.e. voltage standing wave ratio - VSWR) at both input and output ports to thus minimize amplitude and phase variations resulting from impedance mismatching.
Furthermore, at millimeter wave frequencies it is difficult to provide RF circuit components having matched insertion loss and insertion phase characteristics. Thus the cost of procuring RF circuit components having matched insertion loss and insertion phase characteristics and low VSWR characteristics at millimeter wave frequencies may be prohibitively expensive.
Moreover the logistics of keeping the matched sets of RF circuit components together during the assembly of the RF receivers is undesirable. An additional logistic problem arises when one RF circuit component of an RF receiver channel fails. In this case to maintain the desired insertion loss and insertion phase relationship between the three receiver channels, the corresponding components in each of the remaining receiver channels should also be replaced.
A so-called line stretcher (e.g. a transmission line having an adjustable length) provided in each receiver channel may be used to adjust the insertion phase characteristic of each receiver channel. Thus any imbalance between the insertion phase characteristics of one receiver channel relative to the other receiver channels may be removed via the line stretcher.
However, the line stretcher phase matching technique is frequency dependent. Thus this technique provides the receiver channels having a matched phase characteristic over a narrow band of frequencies.
Modern radar systems have relatively broadband receive bandwidths. Thus the line stretcher phase matching approach is not suitable to provide the RF receiver channels having matched insertion phase characteristics over the desired operating frequency bandwidth of broadband radar systems. This .is particularly true in those radar systems which operate in the millimeter wave frequency range. It would be desirable therefore to provide an approach which avoids the problem associated with both the matched components and line stretcher techniques.